<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var age = prompt('输入年龄');
        if (age >= 18) {
            alert("你已经长大了");
        } else if (age < 0) {
            alert("还没有出生");
        } else {
            alert("未成年");
        }
    </script>
</body>

</html>